Each tooth has two proximal sides, usually in contact with respective proximal sides of two adjacent teeth. Class 2 cavities affect one or both of the proximal sides of a tooth. Filling a class 2 cavity typically requires holding a matrix band or strip against the side of the tooth being filled during the filling process in order to contain the filling material, much like a form is used to contain concrete being poured. When silver amalgam was used as the filling material, its density and compactability separated the adjacent teeth far enough to compensate for the thickness of a strip of stainless steel positioned between the teeth that was uniformly of a thickness from 0.0012 inch to 0.002 inch. Therefore, simple monolithic strips of stainless steel were used. With the development of resinous filling materials, the simple relatively thick strips of metal matrix materials of uniform thickness became inadequate, because they tend to leave a gap between the teeth after the strips have been removed.
The initial response of the industry was to increase the separation force produced by a matrix retainer. Forceful matrix retainers were developed to simultaneously hold a sectional matrix between the proximal surfaces of adjacent teeth and to drive the adjacent teeth apart far enough to compensate for the thickness of the matrix. The matrix retainers in such cases are typically split rings that are forcibly opened with a locking type of pliers that is also used to place rubber dam clamps around teeth. Such pliers are commonly known as rubber dam clamp forceps. In one common approach, a partially opened split retention ring is placed between the teeth where the sectional matrix is located with the split ring abutting the matrix. The retention ring is then allowed to close forcibly against the sectional matrix and its adjacent tooth to press the matrix against the tooth being treated. The matrix retainers in common use are all powerful split rings of heavy gauge material (metal or plastic), because, when seated, they must provide enough force to separate the teeth far enough to compensate for the thickness of the sectional matrix, 0.0012 inch to 0.0020 inch. Split rings of this type are made of thick enough material to generate clamping forces of from five to twenty pounds when their ends are separated (spread apart) by a distance that is about equal to the width of a tooth. After the retention ring is removed, the intention is that the previously separated teeth (separated by the retainer) spring back far enough to re-establish a fully contiguous relationship at an area called the contact area.
One problem with these powerful retention rings is that they cannot be used with large fillings. That is, such known retention rings deliver large compressive forces at the sides of the matrix and these forces can indent the sectional matrix or deform it in a portion of the area of the tooth being filled. A second problem with these powerful retention rings is that they are not suitable for use with the selectively thinned matrix materials which are now available (e.g., such as described in U.S. Pat. No. 5,505,618, to Summer; U.S. Pat. No. 6,425,760, to Summer et al.; U.S. Pat. No. 6,509,540, to Summer et al.; and U.S. Pat. No. 6,736,639, to Summer, all of which are incorporated by reference herein), because the forceful compression at the sides of the matrix can deform the matrix.
Another problem with known sectional matrix products and retention rings arises from difficulties in using such products. That is, sectional matrix products are generally placed in a patient's mouth in one step. The powerful retention ring which holds the matrix in position is placed in a second step. It is difficult to hold the matrix in a correct position while the retention ring is being placed around it.